1. Field
Embodiments of the present disclosure relate to manufacturing steel tubes and, in certain embodiments, relate to methods of producing steel tubes for wireline core drilling systems for geological and mining exploration.
2. Description of the Related Art
Steel tubes are used in drill rods for mining exploration. In particular, steel tubes can be used in wireline core drilling systems. The aim of core drilling is to retrieve a core sample, i.e. a long cylinder of rock, which geologists can analyze to determine the composition of the rock under the ground. A wireline core drilling system includes a string of steel tubes (also called rods or pipes) that are joined together (e.g., by threads). The string includes a core barrel at the foot end of the string in a hole. The core barrel includes, at its bottom, a cutting diamond bit. The core barrel also includes an inner tube and an outer tube. When the drilling string rotates, the bit cuts the rock, allowing the core to enter into the inner tube of the core barrel. The core sample is removed from the bottom of the hole through an overshot that is lowered on the end of a wireline. The overshot attaches to the top of the core barrel inner tube and the wireline is pulled back, disengaging the inner tube from the barrel. The inner tube is then hoisted to the surface within the string of drill rods. After the core is removed, the inner tube is dropped down into the outer core barrel and drilling resumes. Therefore, the wireline system does not require the removal of the rod strings for hoisting the core barrel to the surface, as in conventional core drilling, allowing great saving in time.
In particular, seamless or welded steel tubes can be used in drill rods and core barrels. Steel rods can be cast, pierced, and rolled or rolled, formed, and welded to form steel tubes. The steel tubes can go through a number of other processes and heat treatments to form a final product. The standard manufacturing process of this product includes a quenching and tempering at both ends of each tube prior to threading to increase mechanical properties at the ends, as the connection between tubes is integral for mining exploration. Quenching and tempering at the ends of the rods has been utilized as the wall thickness of the tubes may be reduced by almost 50% of the original thickness upon threading of the tube. Therefore, in order to compensate for the loss of material in the tube, the mechanical properties at the ends are increased by the quenching and tempering. Elimination of this process, only at both ends of the bar, would simplify producing a final product.
Steel tubes used as wireline drill rods (WLDR) desire tight dimensional tolerances, i.e. outer diameter and inner diameter consistency, concentricity, and straightness. The reason for these tight dimensional tolerances is two-fold. On one hand, the finished rods, upon manufacturing, have flush connections which are integral for operation. No coupling is used. If the tube geometry does not have the appropriate dimensions, the threading procedure can create tube vibration. Additionally, the threads can be incompletely formed and the tubes can lack the remnant tube wall thickness at the threading. On the other hand, during field operation the WLDR is rotated at a very high speed, up to about 1700 rpm, requiring appropriate concentricity to avoid vibrations in the rod column. Also, a tight dimensional tolerance for the inner diameter is desired to hoist the core barrel in a smooth and uninterrupted way. For these reasons, cold drawn tubes have been used for high performance WLDR. If the tubes are full length quenched and tempered after cold drawing, in order to improve the mechanical properties, dimensional tolerances in the outer and inner diameter are negatively impaired. Therefore, the standard tubes used in the market are cold drawn stress relieved (SR) tubes. The stress relieving heat treatment is performed on the tubes to lower the tube residual stresses. However, the microstructure resulting from a hot rolled and then cold drawn SR tube is substantially ferrite-pearlite with a relatively poor impact toughness. Due to the ferrite-pearlite microstructure formed, WLDR manufacturers are currently forced to quench and temper both tube ends at the location where the threads are going to be machined in order to improve the mechanical properties in these critical zones. End quenching and tempering is a critical, yet expensive, operation. Also, the tube body remains with the original ferrite-pearlite microstructure with poor impact toughness. Field failures occur due to the ferrite-pearlite microstructure within the tube body. In some cases, indentations produced by machine gripping propagate a long crack that has not arrested, therefore producing a high severity failure mode. On top of that, there is a strong limitation in the mechanical strength that can be achieved through cold drawing. Therefore, the abrasion resistance of WLDR at the tube body is relatively poor, and many rods have to be scrapped before the expected rod life.
The conditions for operating mining exploration are very demanding. Steel tubes used in mining exploration are affected by, at least, torsion forces, tension forces, and bending forces. Due to the demanding stresses imposed on the steel tubes, preferred standard properties for drill rods are a yield strength of at least about 620 MPa, an ultimate tensile strength of at least about 724 MPa, and an elongation of at least 15%. For rods currently on the market, the main deficiencies are low toughness, relatively low hardness, and weak mechanical properties.
High abrasion resistance is therefore desirable for steel tubes for drill rods as well as good mechanical properties such as high impact toughness while maintaining good dimensional tolerances. As such, there is a need to improve these properties over conventional steel tubes.